[Gb-ccb] 17feb04 CCB telecon minutes

[John Ford]

extra complexity invites extra problems.  I suggest we trust the
reference signals.

Martin Shepherd writes:
 > 
 > 
 > On Thu, 19 Feb 2004, John Ford wrote:
 > >...
 > > The phase is adjusted locally to the receiver room, just upstream of
 > > the CCB.  The 1 PPS and 10 MHz signals that the CCB sees are phase
 > > locked together and the edges are noncoincident.  If you wanted to
 > > change the phase, just change the length of one of the cables.
 > 
 > Wouldn't it make sense to force correct operation within the CCB,
 > rather than rely on subtle external conditions like this? If the wrong
 > peice of coax got used during an observing run, how would one ever
 > know? Wouldn't it be prudent to add a flip-flop outside the FPGAs,
 > that generates a 1-PPS pulse that is explicitly delayed to the
 > opposite edge of the 10MHz signal to the edge that is used to
 > synchronize operations within the FPGAs? This would ensure that all of
 > the FPGAs received a 1PPS edge that always occurred 50ns before the
 > next synchronization edge of the 10MHz reference signal. There would
 > then be no ambiguity in which 10MHz cycle to sync to, and no
 > dependence on using the right external cable.
 > 
 > Martin
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